Hi clarkterri, welcome to Cruise-Chat. I wouldn't book anything before noon. The ship may dock at 6:30am, but it won't be cleared until around 8:30-9am. Then you will have to wait to be called, unless you do self-debarkation. No reason to hurry on your return. You can have breakfast, then debark, gather your luggage, maybe take a tour of San Juan, or head to the airport.
Hope you have a wonderful cruise! Best wishes!